1. Technical Field
This invention relates to cordless telephones and more particularly to cordless telephone base units arranged for deployment in a public telephone system.
2. Description of the Prior Art
Telephone communications in public areas have traditionally been limited to a person using a public telephone where he or she could make either charge calls, pay calls or credit card calls. The mobility of the user has thus been limited to the immediate surroundings of the public telephone, as determined by the length of the telephone handset cord. Other alternatives, such as cellular telephone service and the new telepoint system, are expensive and therefore unattractive to many potential users.
One economical alternative to the public telephone is a public cordless telephone system. Such a system is described in U.S. Pat. No. 4,953,198 which issued to J. J. Daly et al. on Aug. 28, 1990. The public cordless telephone system includes a plurality of public base units that are accessible by a plurality of portable, or cordless, public handset units. Each one of these handset units is capable of establishing communications with each one of the base units over a plurality of predetermined communication channels. Communication over each one of these channels is by way of a radio frequency carrier signal that is modulated by control signals and audio signals to and from a public base unit as appropriate. And the public base units are connected to telephone lines for providing conventional telephone communications for the public handset units.
In the operation of the public cordless telephone system, establishing a communications link between the public handset unit and the public base unit is initiated by a user activating the handset unit. Once activated, the handset unit, in turn, interrogates each one of the plurality of predetermined communication channels in order to establish communications with a public base unit on a nonbusy, or available, one of these channels. Once the handset unit selects an available channel, it transmits a service request which includes an identification code over this channel. A public base unit within the reception range of the handset unit receives the handset unit's identification code, appends its own identification code to this received code and then transmits the combined handset unit identification code and public base unit identification code back to the handset unit. The handset unit receives this combined code from the public base unit and then compares the handset portion of the received code with the code that it previously transmitted. If a favorable comparison of this code and the handset portion of the code received from the public base unit is obtained, the communications link is then established between the handset unit and the public base unit on the selected communication channel.
In the United States, all cordless telephones currently are required to operate within the same allocated frequency spectrum. Thus the plurality of predetermined communication channels used by the public base units and public handset units in the public cordless system also are used by those base units and associated handset units employed in residential cordless telephone sets. As cordless telephone sets, in general, and both public base units and public handset units, in particular, increase in number, some service attempts initiated by the user of a public handset unit possibly will not be completed because the public handset unit will be unable to find a nonbusy, or available, communication channel on which to transmit its service request. Moreover, some service attempts initiated by the user of a public handset unit possibly will not be completed because all the public base units within reception range of the handset unit are then busy. Unfortunately, a user of a public handset unit that finds all the communication channels occupied is unable to determine whether he or she is within the reception range of a public base unit that is then busy or whether the communication channels are all occupied by residential base units communicating with their associated handset units. Also a user of the public handset unit that finds a communication channel available but does not receive a response from a public base unit to its service request is similarly unable to determine whether the handset unit is within the reception range of a public base unit that is then busy or whether the handset unit is outside of the reception range of all public base units.